An interferometer is a device in which an optical interference pattern of fringes is formed and used to make measurements, typically related to the wavelength of the optical source. Interferometers may be used for monitoring relative wavelength changes of an optical source. In such wavelength-monitoring applications, interferometers may also be referred to as “wavecounters” or “wavemeters”.
“Fiber interferometers” are interferometers constructed entirely using fiber-optic waveguides and fiber-optic components. FIG. 1 diagrammatically shows typical prior art fiber interferometer 100, which is based on a well known Michelson architecture. Interferometer 100 comprises fiber directional coupler 103 and Faraday rotator mirrors 106 and 107. Coupler leads 102 and 108 are connected to light source 101 and detector 109, respectively. Faraday rotator mirrors 106 and 107 are attached to coupler fiber leads 104 and 105. Some additional fiber (115) may also be spliced between coupler lead 104 (or 105), and Faraday mirrors 106 (or 107). The Faraday mirrors 106 and 107 reflect light from light source 101 back through arms 104 and 105, and these reflected signals are also exchanged and combined at coupling 103 and carried back through lead 108 to be detected by the optical sensor 109.
One of the main difficulties in using fiber interferometers is that, compared to free-space interferometers, they are more sensitive to changes in temperature. This thermal sensitivity arises because lengths of fiber expand appreciably with increasing temperature, while free-space, or air, expands comparably much less. Since stable operation of an interferometer depends on path length differences being stable, this thermal expansion of the fiber problematically causes thermal drift of the interferogram measurement.
Arm 120 of interferometer 100 comprises fibers 104 and 115 and lead 116 of Faraday mirror 106. Arm 121 comprises fiber 105 and lead 117 of Faraday mirror 107. The length of these arms, which are functions of the ambient temperature T, may be respectively designated as L1(T) and L2(T). Typically, all of the fiber used in interferometer 100 is of a uniform variety or construction, as is consistent with prior art interferometers. This fiber possesses a coefficient of thermal expansion (CTE) of α parts-per-million per degree Celsius (ppm/° C.), or equivalently, parts-per-million per degree Kelvin (ppm/° K). Based on thermal expansion theory:L1(T)=(1+αT)·L1(0)  (1)L2(T)=(1+αT)·L2(0)  (2)where L1(0) and L2(0) are the lengths of the interferometer arms 120 and 121 extrapolated back to a temperature of zero degrees Kelvin. These equations show how each arm's length changes in direct proportion to the change in ambient temperature.
Thus, the path-length-difference of the two arms as a function of temperature, is:ΔL=L2(T)−L1(T)  (3)which indicates that ΔL, the path length difference between interferometer arms 120 and 121, is a function of the temperature T. Since ΔL is a function of temperature, the interferometer's output may drift with temperature.